When
wild populations of certain species in nature reach extremely low number
of individuals faces imminent danger of extinction.

Nowadays
in the presence of the demographic limitative factors, the only approach
to save living creatures is to bring them into captivity under a breeding
program in controlled environment, hoping this way build up the
effective’s number, and try to re-establish the wild population on
natural habitat later.

In
captivity urgent actions are imperative to maintain reproductive viability,
in particular among species facing threat or danger of extinction in their
original ecosystems. It becomes necessary invest in a process to reach
auto sustained level for a population under this conditions can survival.

On
a different stage, very far away from domestic aquarium fish, the process
is being tried with endangered species, but it involves many challenges,
both biological and political.

Private
fish keeping rarely can put together all the proper conditions and space
to a population sustainable level, unless cooperation between quite a few
holders of the same specie.

Several
problems arise when a species reaches a very small population size,
specially this :

1)
Loss of genetic variability

Depending
on the specie we are talking about; when a fish population drops below a
number with reference to something like 100 breeding individuals, the
amount of genetic dissimilarity is reduced, and these can lead to eventual
diminish of immunity capacity as to adaptation to environmental changes.

Some
livebearer Cyprinodontiformes are genetically extremely uniform.

This
indicates that they are hereditarily identical as far as genes are
concerned, even if they are not relatives.

The immune systems of all animals are absolutely dependent on genetic
diversity.

There are probably millions of genes linked to immunity, each one coded for a specific antibody
or cell surface receptor.

The
problem arises when there isn’t enough information about these genes at
chromosomes.

Small groups raised in aquarium are not the only cause to loss of genetic
variability.

When somewhere at an evolutionary process an event outcomes and a
significant percentage of certain specie is killed, or otherwise prevented
from reproducing, the reduced survival or breeding group is facing what
scientists call a population bottleneck or genetic bottleneck.

At this moment the sustainability of that specie will be deeply affected.

Genetic variability can often be evaluated by gene diversity (
allelic richness and diversity, but also heterozygosity ).

2)
Inbreeding

Populations
below about 50 breeding individuals are forced to breed with close
relatives.

The
pattern of breeding between genetically so close fish is called inbreeding.

As
end result, this can lead to depression of fitness called inbreeding
depression, which includes lethal consequences to that population
beginning with decreased fertility and fecundity, high mortality rate and birth defects.

Aquarium
populations which went down to less than 30 individuals, have to be
managed very carefully in order to minimize inbreeding deadly effects.

In
certain captive populations may well show inbreeding symptoms, even among
non-related fish, precisely because some Livebearer Cyprinodontiformes are
indeed genetically very homogeneous by nature and are more vulnerable to
reduced populations keeping.

It
is almost impossible to determine how far reduced populations keeping will
affect particular species, as there is more or less susceptibility to this
problem from one to another.

3)
Selective breeding and induced hybridizing

When under man made breeding selection many species are equally submitted
to loss of genetic variability but above all to inbreeding.

Searching for improvement or strengthening of specific characters, there
are significant risks of genetically transmitted anomalies to be
reinforced on the process in the same way.

With
long periods in captivity there is a genuine danger that unintentional
selective breeding can result in the establishment of traits that are
desirable in captivity but opposed to natural selection.

Aquarium
conservation may be seen as apparently the most viable way to save in
danger of extinction species in the so called « ex situ conservation »,
but unnatural strain selection may endanger definitively this last survival
members before an eventual return to the wild.

Besides
man made selection to obtain a specific strain, forced hybridizing must be
considered as risk factor.

Take
for an example, that endangered species of which all the remaining individuals
of the same sex were crossed with
what was thought to be their closest relative.

Theoreticaly
it was possible to produce hybrids that, could be interbred to give back a strain of the threatened
species that could stay close to 100% “ pure “ of the original.

Unfortunately, almost all
this events have failed either by bad choices on the genetically closest
specie but also by unpredictability of the genetic course.

Natural
hybridization seems to be unusual. However it con occur oftenly when species are
reduced to very small numbers.

Non
natural fertil hybrids in the wild can become a menace to parental species
biodiversity and survival as independent forms of live.

Hybrid vigor can
soon overcome breeding chances wile competing with members of original
species.

We
should be extremely gratified and thank you in advance if some one
could provide new data about this topic, or even eventually any
correction to be made on this document. For
this purpose please be so kind and write us.

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